Melt butter in large skillet over medium heat. Add mushrooms; cook until softened, 6 to 7 minutes, stirring occasionally. Sprinkle flour over mushrooms; cook, stirring, 1 minute.
Gradually add chicken broth, stirring constantly, scraping any browned bits from bottom of skillet. Bring to boiling. Reduce heat; simmer until sauce is thickened, about 5 minutes.
Add salt, garlic powder, nutmeg and pepper. Remove from heat. Stir in mustard. Add chicken.
Pair Chunky Chicken Dijon with your choice of Veggie Blend and Starch.
Bake in 400°F oven 30 minutes or until heated through.